1. Field of the Invention
This invention relates to a protective device for a load-control device that has a CPU which controls the load based on signals from sensors and more specifically for a load-control device wherein the power supply for said CPU and sensors is obtained from a DC voltage-regulation circuit which drops the output voltage of an external power supply.
2. Description of the Related Art
An ECU (electronic control unit) is often used to control the engine of a vehicle. It usually comprises a CPU, an analog/digital (A/D) converter, and an input/output interface, and generally controls actuators for elements such as an injector and an ignitor, based on information from a sensor group (such as manifold pressure sensor, water temperature sensor, and throttle position sensor) with regard to engine operation and on a preset program.
In such an arrangement, in the past, as shown in the simplified drawing of FIG. 8, it has been usual, (in addition to providing within ECU 1 a DC voltage -regulation circuit 3, which supplies power to CPU 2 and additionally to a group of digital ICs not shown in the drawing,) to obtain power for sensor group 4 as well from the output line Vcc of DC voltage-regulation circuit 3.
In the above-described configuration, sometimes the lead wires will make a connection between ECU 1 and sensor group 4, because of miswiring or because said lead wires come into contact either with part of the vehicle body at ground potential, causing the Vcc line to drop to ground potential, or with the power lead from the vehicle battery, causing Vcc to be raised to the supply voltage of the vehicle battery (generally 12 V). Such a situation causes the problem described below.
Essentially, if the output line Vcc is dropped to the ground level, because the power supply to CPU 2 and other digital ICs are cut off, ECU 1 stops functioning, and engine control becomes entirely impossible. Also, on occasion the output line Vcc is raised to the supply voltage from the vehicle battery and a voltage exceeding the allowable input voltage protection value is input to CPU 2 and other elements (generally approximately 7 V). This causes a great danger of permanent damage to the IC group that includes the CPU 2, thus making ECU 1 itself unusable.
A disclosure was made, in Japanese Unexamined Utility Model 1-135321, of a solution to this type of problem. However, the technology disclosed in this publication consists of connecting a resistance in series with a sensor, and when the voltage across the resistance becomes large due to a ground short, turning on a transistor and cutting off the power supply to the sensor.
Thus, in the case in which the voltage across the resistance fluctuates about the voltage at which the transistor is turned on and off, the power supply to the sensor is applied and cut off repeatedly, so that the signal from the sensor is maintained at a level which is not judged to be abnormal. When setting the electrical levels for vehicle control based on information from the sensor signal, there is a danger that a misoperation of the vehicle will result.